1. Field of the Invention
The present invention relates to a semiconductor apparatus and, particularly, to a semiconductor apparatus including a voltage divider that generates a divided voltage by dividing a voltage difference between a first power supply and a second power supply.
2. Description of Related Art
As a recent trend, a semiconductor apparatus generates a voltage different from a power supply voltage inside the apparatus and makes an internal circuit operate with the generated voltage in order to improve the performance. An example of such a voltage is a bias voltage (or a reference voltage). The bias voltage is a voltage that serves as a reference of a circuit operation and it is required to maintain a stable voltage without depending on variations of a power supply voltage. An example of a bias voltage generator is disclosed in Japanese Unexamined Patent Application Publication No. 2003-173213.
FIG. 15 shows a block diagram of a bias voltage generator 100 disclosed in Japanese Unexamined Patent Application Publication No. 2003-173213. Referring to FIG. 15, the bias voltage generator 100 includes a resistor element R40, an output transistor NT40, and a current control circuit 140. The resistor element R40 and the output transistor NT40 are connected in series between a power supply VCC and a power supply VSS. In the bias voltage generator 100, a node between the resistor element R40 and the output transistor NT40 serves as an output node N10. A gate and a drain of the output transistor NT40 are connected to each other. The current control circuit 140 includes a resistor element R41 and transistors NT41 and NT42. The current control circuit 140 is placed in parallel with the output transistor NT40. The current control circuit 140 increases a current I12 that flows from the output node N10 to the power supply VSS as the voltage of the power supply VCC increases. The bias voltage generator 100 can thereby allow the value of a bias voltage VBS that is output from the output node N10 to remain substantially constant or decrease with respect to an increase in the voltage of the power supply VCC.
However, in the bias voltage generator 100, a voltage level that serves as a reference of the bias voltage VBS is determined by a threshold voltage (or a diode voltage) of the output transistor NT40 with respect to the power supply VSS. Some semiconductor apparatus generates a negative voltage inside the apparatus and uses it for a circuit in the apparatus in order to improve the performance. In the case of using the bias voltage generator 100 in such a case, the bias voltage VBS varies with variations of the negative voltage, and therefore the bias voltage generator 100 cannot be used for a circuit in which both the power supply VCC and the power supply VSS vary.
DRAM (Dynamic Random Access Memory) is one of such semiconductor apparatus using a negative voltage. In DRAM, a negative voltage is generated for a substrate voltage VBB and used as a substrate voltage of a transistor of a memory cell. By using the negative voltage as the substrate voltage VBB, it is possible in the DRAM to reduce a leakage current of the memory cell and improve data storage capability (cf. “Very LSI Memory” written by Kiyoo Itoh and published from Baihukan). The negative voltage is generated typically by a charge pump. An example of a negative voltage generator that generates a negative voltage with use of a charge pump is disclosed in Japanese Unexamined Patent Application Publication No. 11-150230.
FIG. 16 shows a block diagram of a negative voltage generator 200 disclosed in Japanese Unexamined Patent Application Publication No. 11-150230. Referring to FIG. 16, the negative voltage generator 200 includes a reference voltage generator 210, a voltage divider 220, a comparator 230, an oscillator 240, a clock buffer 250, and a negative voltage charge pump 260. The reference voltage generator 210 generates a constant voltage having a voltage value between a power supply voltage and a ground voltage and outputs it as a reference voltage 201. The voltage divider 220 divides a voltage between the reference voltage 201 and a negative voltage 204 and outputs it as a divided voltage 202. In the voltage divider 220, a voltage dividing ratio is set in such a way that the divided voltage 202 becomes a ground voltage when the negative voltage 204 reaches a voltage value to be set. The comparator 230 compares a voltage value of the divided voltage 202 with the ground voltage, and if the voltage value of the divided voltage 202 is higher than the ground voltage, it activates an oscillator control signal 203, and if the voltage value of the divided voltage 202 is lower than the ground voltage, it inactivates the oscillator control signal 203. The oscillator 240 outputs a first oscillator output signal 205 and a second oscillator output signal 206 having opposite phases when the oscillator control signal 203 is activated. The clock buffer 250 outputs a first complementary pulse signal 207 corresponding to the first oscillator output signal 205, and outputs a second complementary pulse signal 208 corresponding to the second oscillator output signal 206. The negative voltage charge pump 260 outputs the negative voltage 204 based on the first complementary pulse signal 207 and the second complementary pulse signal 208.
The negative voltage generator 200 has a circuit configuration in which, when the negative voltage 204 reaches a preset voltage, the divided voltage 202 becomes the ground voltage that is input as a comparative voltage of the comparator 230. In the negative voltage generator 200, the reference voltage generator 210 generates the reference voltage 201 that is not dependent on variations of the power supply voltage VCC in order to stably obtain the negative voltage 204 even when the power supply voltage VCC varies. Further, the voltage divider 220 determines a voltage dividing ratio between the reference voltage 201 and the negative voltage 204 by a resistor string. Specifically, in the negative voltage generator 200, the divided voltage 202 that reflects the voltage value of the negative voltage 204 is generated based on the stable reference voltage 201 and a fixed voltage dividing ratio. The negative voltage generator 200 can thereby stabilize the voltage value of the negative voltage 204 regardless of the power supply voltage VCC.